1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery and a method of manufacturing the battery, more particularly to a heat-resistant insulating porous membrane provided on the surface of the negative electrode, positive electrode, or separator thereof.
2. Background Art
With the advancement of portable and cordless electronic equipment, there are growing expectations for smaller and more lightweight non-aqueous electrolyte secondary batteries having higher energy density. For such batteries predominated by lithium secondary batteries, their high energy density necessitates considerations of the design for improving safety. Particularly when a micro-porous membrane made of a polyolefin resin, such as polyethylene and polypropylene, is used as a separator, the membrane is likely to shrink and cause short circuits even at temperatures up to 150° C. In addition, when a sharp projection, such as a nail, penetrates through the battery, heat instantaneously generated by short circuits further develops the short-circuited area. Further, reaction heat added thereto can cause an abnormally high-temperature state of the battery.
Proposed to address this problem is a technique in which a porous membrane containing an inorganic filler and binder is provided on the surface of the positive or negative electrode of a battery. This membrane prevents direct contact between the positive and negative electrodes and further increases in the temperature of the battery even when the battery is at an abnormally high temperature and a minute portion of the separator is broken. In formation of such a micro-porous membrane, an inorganic filler and a binder are kneaded in the existence of a dispersion medium to provide a paste, and the paste is applied to the surface of the positive or negative electrode, and dried. Such a proposal is disclosed in Japanese Patent Unexamined Publication No. 2005-222780.
However, oxides, such as aluminum oxide, titanium oxide, and magnesium oxide, used as inorganic fillers are likely to agglomerate. Further, such oxides have hydrophilic terminal groups, such as hydroxyl groups, on the surface thereof, and thus the oxides are likely to adsorb moisture. For these reasons, such oxides are more likely to agglomerate through water molecules adsorbed in the existence of moisture. Thus, when such a material is used as an inorganic filler, the inorganic filler agglomerates in preparation of a paste and the dispersibility of the inorganic filler in the paste deteriorates. In this manner, the use of a paste prepared with the agglomerated inorganic filler makes the thickness of the porous membrane non-uniform. This causes variations in the distance between the positive and negative electrodes, and non-uniformity of battery reaction, and gives influences on battery characteristics, such as deterioration of charge-discharge cycle characteristics. When the porous membrane has an extremely non-uniform thickness, and a minute portion of the separator is broken by heat generation, a thin portion of the membrane is broken by thermal expansion of the positive or negative electrode and cannot prevent direct contact between the positive and negative electrodes. Thus, the porous membrane cannot accomplish the intended purpose thereof.